The present invention relates to improvements in an optical fiber cable structure and, more particularly, to an optical fiber cable which is effective when laid on the bottom of the water or sea.
With the improvement of the optical fiber structure for a reduced transmission loss and with the enhancement of the characteristics of optical devices such as a semiconductor laser and so on, optical communication is now making a great advance towards its practical application. Under such a situation, the elevation of the reliability of each part constituting a fiber optic communication system is an important problem for its practical application; in particular, the improvement of the long-term stability of the transmission characteristic of the optical fiber is indispensable.
Heretofore, it has been considered that the transmission loss by the optical fiber is fluctuated by its micro-bending or ambient temperature variations but undergoes substantially no variations with the lapse of time. Therefore, a main problem of conventional optical fiber cables is how optical fibers, which are very fragile mechanically, are protected from any external pressure such as a water pressure or tension, and various cable structures have been proposed.
In various experiments conducted by the present inventor on variations in the transmission characteristics by water entering into optical fiber cables in consideration of the fact that the optical fiber cables are laid on a river bed or sea bed under a high water pressure and may sometimes be broken by fishing implements, an anchor of a fishing boat and so forth, it has been found that when water diffuses into the optical fiber cable, a sharp light absorbing spectrum for a particular wavelength appears in a short time. It has also been ascertained that the higher the water pressure is, the larger the light absorbing spectrum becomes. This phenomenon is a fatal defect of a system which is required to have a design lifetime of more than 20 years, such as an international fiber optic communication system.